1. Field of the Invention
The present invention relates to a sewing machine for sewing a workpiece while feeding the workpiece with an upper feeding foot, a presser foot, and feeding teeth.
2. Description of the Related Art
In a sewing machine, as shown in FIG. 21A, when a needle 101 moves down and pierces a workpiece, an upper feeding foot 104 moves down at the same time. The upper feeding foot 104 holds the workpiece with feed teeth 105 which move up in its substantially elliptical oscillation motion.
Next, as shown in FIG. 21B, when the upper feeding foot 104 presses the workpiece, a presser foot 106 which previously presses the workpiece starts to move up from the workpiece. At this time, the needle 101 piercing the workpiece, and the upper feeding foot 104 and the feeding teeth holding the workpiece oscillate in a left direction of the drawing, whereby the workpiece is fed in the left direction.
As shown in FIG. 21C, when the upper feeding foot 104 and the feeding teeth 105 finishes to cooperatively feed the workpiece, the needle 101 moves up and starts to be drawn from the workpiece. At the same time, the presser foot 105 moves down and presses the workpiece.
As shown in FIG. 21D, simultaneously with the presser foot 106 pressing the workpiece, the upper feeding foot 104 and the feeding teeth 105 start to leave from the workpiece. In the meantime, the needle 101, the upper feeding foot 104 and the feeding teeth 105 oscillate and leaves from the workpiece, thereby returning to their initial positions as shown in FIG. 21A. The workpiece is fed by repeating these operations.
In starting a sewing operation, a thread needs to be drawn to a back side of the workpiece. However, if the thread is stepped on by the presser foot 106, it is difficult to draw the thread. For this reason, the presser foot 106 is moved up when starting the sewing operation in order to easily draw the thread. More specifically, as shown in FIG. 22 and FIG. 23, the presser foot 106 is held by a holding bar 107. An upper portion of the holding bar 107 is connected to a holding bar bracket 108, and is engaged with a spring 111. A front end portion of a lift lever 109 comes in contact with a lower face of one end portion of the holding bar bracket 108. The lift lever 109 is substantially in an L shape, and moves up the holding bar 107 and the presser foot 106 by lifting the holding bar bracket 108. A corner portion of the lift lever 109 is rotatably supported so that the front end portion of the lift lever 109 moves up and down. An air cylinder 110A is connected to a base end portion of the lift lever 109, and the lift lever 109 rotates around the corner portion by moving a piston rod of the air cylinder 110 back and forth. When the piston rod of the air cylinder 110 moves forward from a state shown in FIG. 22, the lift lever 109 rotates in a clockwise direction, whereby the presser foot 106 moves down by a biasing force of the spring 111 and changes to a state shown in FIG. 23. On the other hand, when the piston rod of the air cylinder 110 moves rearward from the state shown in FIG. 23, the lift lever 109 rotates in a counterclockwise direction and changes to the state shown in FIG. 22, whereby the presser foot 106 moves up. Namely, the presser foot is prevented from stepping on the thread when drawing the thread by moving the piston rod of the air cylinder 110 back and forth at a timing of starting the sewing operation.
When a thickness of the workpiece varies, a height where the presser foot 106 presses the workpiece varies. When there is such a variation, even if the lift lever 109 is rotated in the clockwise direction so as to be moved back to its initial position by moving the piston rod of the air cylinder 110 forward after the thread is drawn by a shuttle, depending on the thickness of the workpiece, there is a case in which the presser foot 106 is not moved down to its initial position, whereby a gap L is generated between the lift lever 109 and the holding bar bracket 108.
When there is such a gap L, a time loss is caused between operations of the air cylinder 110 and the presser foot 106, resulting in a variation in timings of moving up the presser foot. In order to solve this problem, a potentiometer may be attached to the lift lever 109 so that a rotational angle of the lift lever 109 is detected by the potentiometer. If the air cylinder 110 is controlled based on a result of the detection, the gap L can be compensated and the presser foot can be moved up at optimum timings (see, e.g., JP-A-2004-057822).
However, in recent years, in view of restraining a manufacturing cost, a reduction in the number of components and an improvement in controlling efficiency are desired. This is not exceptional for a structure with regard to the moving up operation of the presser foot. For example, if the moving up operation of the presser foot with stabilized timings can be realized without the potentiometer, the structure and the control can be simplified.